The present invention relates to a module for collimating a light beam.
A module of this type is known, for example, from U.S. Pat. No. 4,698,730 which describes a module comprising an LED with a radial-type package, mounted on a support, and an optical element operating with total internal reflection. The optical element has a substantially cylindrical recess in which the lens which acts as a package for the LED is housed. The device is characterized in that part of the beam emitted by the LED is collimated by the lens which constitutes its package whilst another portion of the beam is collimated by a reflector of substantially parabolic cross-section.
Other solutions similar to this have been proposed, for example, in patent application WO00/24062, in which the collimation function is performed by a transparent dielectric module which houses the LED source in a suitable, substantially cylindrical recess; as in the previous case, a portion of the beam is collimated by a reflector of substantially parabolic cross-section and operating with total internal reflection whilst a second portion is collimated by a lens the first surface of which is constituted by the upper surface of the recess.
Further variations of the same concept are put forward in patent applications EP 0 798 788, DE 195 07 234, WO00/36336, and WO03/048637.
In some applications, the devices described above have limited versatility. Various solutions for producing optical units which use solid-state light sources, in particular LEDs, are under investigation in the automotive sector. In these applications, particularly with regard to headlights with a dipping function, the light beams projected must satisfy certain requirements which are imposed by the standards that are in force on the subject.
In the case of dipped headlights, the divergence of the beam projected is particularly critical for the regions of the headlight which project the light towards the zone of the distribution that is close to the horizon (see, for example, FIG. 1) where the standard provides for a very sharp transition from the maximum or peak of the distribution, at an angle of 1-2 degrees below the horizon and intensity values close to zero above the horizon line. For dipped headlights according to the European standard, the distribution of luminous intensity adopts the characteristic form shown in FIG. 1 in which the lines join points of equal luminous intensity; the demarcation line in the region of the horizon is known as the cut-off line. In the European dipped beam, the cut-off line has an indentation on the right-hand side, forming an angle of about 15 degrees with the axis of the horizon. This indentation is absent in the American dipped beam and in the UK and Japan it is reversed horizontally.
Owing to the particular structure of the collimator used, the devices described above do not permit the production of optical units in which the light distribution produced can be regulated precisely in order to adapt it to the different patterns of illumination required by the standards. Moreover, in all of the solutions described above, the focal length of the lens (operating on a portion of the beam emitted by the LED) must be kept to the minimum if an excessive increase in the dimensions of the module is to be avoided; since the divergence θ of the beam emerging from the collimator is generally determined by the linear extent of the source (d) and by the focal length (f), by the equation θ=arctan(d/f), the solutions described above do not enable the divergence to be reduced below a threshold value, obtaining the cut-off specified, without an excessive increase in the dimensions of the module.
There are also known headlights which, in order to obtain the cut-off in the distribution, use a so-called poly-ellipsoidal reflector configuration, as shown schematically in FIG. 2. In accordance with this configuration, a support plate P of a light source S also acts as a diaphragm for screening some of the light radiation reflected by a reflecting surface R with an elliptical profile. The emerging radiation is then refracted by a lens L.
The limitation of this configuration is its low efficiency owing to the presence of the diaphragm which absorbs some of the light radiation focused by the poly-ellipsoidal reflector.